Nuclear Magnetic Resonance Spectroscopy:

Nuclear magnetic resonance (NMR) spectroscopy is concerned with the transitions between different nuclear spin states of certain nuclei obtained by placing the sample in an external magnetic field. The information obtained from NMR spectrum can be used to decipher the structure of molecules.

Any nucleus with a spin angular momentum quantum number, I ≠ 0, corresponds to a spinning positive charge and is associated with a magnetic moment (µ) i.e., these act as tiny bar magnets. When placed in an applied field, these nuclei can take up any of the (2I + 1) orientations with respect to the direction of the applied field. For a nucleus with I = ½ it can take up only two orientations. These are aligned either with or against the field; these correspond to two different energy levels. The available spins distribute themselves between these spin states according to the Boltzmann distribution. In addition to aligning with or against the field these also start precessing around the direction of the applied filed. The energy separation of the two levels and the frequency of precession are proportional to the applied field. Incidentally, the frequency required causing a transition between these levels and the precessional frequency is identical.